Heterogeneous materials, such as heterogeneous solid materials, occur naturally and may also be formed by man-made processes. For example, naturally occurring ores may include volumes containing a material of interest (i.e., a so-called “bearing fraction”), such as a metal or a mineral, mixed with volumes not containing the material of interest (i.e., a so-called “non-bearing fraction”). Recovery of the material of interest generally requires physical or chemical separation of the bearing fraction from the non-bearing fraction. Chemical separation may require reagents (e.g., cyanide, acids, carbonates), which may be expensive or raise environmental challenges.
As one example of a heterogeneous material, uranium is typically found in nature as uranium ore. Low-grade uranium ore may contain any form of uranium-containing compounds in concentrations up to about 5 lbs of U3O8 equivalent per ton of ore (about 2.5 kg of U3O8 equivalent per 1000 kg of ore, or about 0.25% uranium oxides by weight), whereas higher grade ore may contain uranium-containing compounds in concentrations of about 8 lbs of U3O8 equivalent per ton of ore (about 4.0 kg of U3O8 equivalent per 1000 kg of ore, or about 0.4% uranium oxides by weight), about 30 lbs of U3O8 equivalent per ton of ore (about 15 kg of U3O8 equivalent per 1000 kg of ore, or about 1.5% uranium oxides by weight) or more.
Uranium deposits may be formed in sandstone by erosion and redeposition. For example, an uplift may raise a uranium-bearing source rock and expose the source rock to the atmosphere. The source rock may then erode, forming solutions of uranium and secondary minerals. The solutions may migrate along the surface of the earth or through permeable subsurface channels into a sandstone formation, stopping at a structural or chemical boundary. Uranium minerals may then be deposited as a patina or coating around or between grains of the formation. Uranium may also be present in carbonaceous materials within sandstone. Uranium may be all or a portion of the cementing material between grains of the formation.
FIG. 1 shows a photomicrograph of a section of a sandstone formations from the Shirley Basin in Central Wyoming. As shown in FIG. 1, uranium-bearing sandstone 10 may include various constituents. In general, oversize material 12 may be defined as relatively large particles or fragments, such as homogeneous particles of host rock. Oversize material 12 may also be defined as particles larger than can be processed in a particular processing system. For example, in some sandstone 10, oversize material 12 may include cobbles and stones arbitrarily defined as material having an average diameter larger than about 0.25 inches (in.) (6.35 mm). Oversize materials 12 in sandstone 10 generally do not contain much uranium. Grains 14 may generally be defined as particles or fragments smaller than oversize material 12. Grains 14 may include particles having diameters from about 400-mesh (i.e., about 0.0015 in. or about 0.037 mm) to about 0.25 in. (6.35 mm), and may include quartz or feldspar. Grains 14 in sandstone 10 do not typically contain much uranium, but uranium may be formed around the grains 14 due to deposition. Fines may be generally defined as particles disposed among the oversize material 12 and the grains 14, and may include materials also found in the grains 14 and oversized material 12, such as uranium, quartz, feldspar, etc. Fines may cement the oversize material 12 and the grains 14 into a solid mass. Fines in uranium-bearing sandstone 10 (e.g., particles smaller than about 400-mesh) may include light fines 16 and heavy fines 18. Light fines 16 generally have a specific gravity up to about 4.0 with reference to water, whereas heavy fines 18 have a specific gravity greater than about 4.0. Uranium compounds are generally components of the heavy fines 18, but may also be a part of light fines 16 in the form of deposits on carbonaceous materials. For example, uraninite has a specific gravity from about 6.5 to about 10.95, depending on its degree of oxidation, and coffinite has a specific gravity of about 5.4. Both light fines 16 and heavy fines 18 may be bound to grains 14 in the sandstone 10. In the sandstone 10, the oversize material 12, grains 14, light fines 16, and heavy fines 18 may be combined into a single mass.
Uranium may conventionally be recovered through in-situ recovery (ISR), also known in the art as in-situ leaching (ISL) or solution mining. In ISR, a leachate or lixiviant solution is pumped into an ore formation through a well. The solution permeates the formation and dissolves a portion of the ore. The solution is extracted through another well and processed to recover the uranium. Reagents used to dissolve uranium of the ore may include an acid or carbonate. ISR may have various environmental and operational concerns, such as mobilization of uranium or heavy metals into aquifers, footprint of surface operations, interconnection of wells, etc. ISR typically requires particular reagents, which must be supplied, recovered, and treated. Because ISR relies on the subsurface transport of a solution, ISR cannot generally be used in formations that are impermeable or shallow.
Uranium may also conventionally be mined in underground mines or surface mines (e.g., strip mines, open-pit mines, etc.). During such mining activities, it may be necessary to process large quantities of material having a concentration of uranium too low for economic recovery by conventional processes. Such material (e.g., overburden) may be treated as waste or as a material for use in mine reclamation. Conventional mining may produce significant amounts of such low-concentration material, which may require treatment during or subsequent to mining operations. It would therefore be advantageous to provide a method of uranium recovery that minimizes or alleviates these concerns.